Brij Bhushan Kabra was considered an expert in which of the following instruments in the field of Indian classical music?

The correct answer is Guitar.

Step-by-Step Explanation:
1. Introduction of Brij Bhushan Kabra: Brij Bhushan Kabra (1937–2018) was a pioneering Indian musician who became famous for introducing and adapting a Western instrument into the strict framework of Hindustani classical music.
2. Modification of the Instrument: He took the Western acoustic guitar (specifically the Hawaiian slide guitar) and modified it to play Indian classical ragas. To achieve the slides (meend) and microtones (shrutis) essential to Indian classical music, he played the guitar flat on his lap using a metal slide, and added sympathetic and drone (chikari) strings similar to those found on a sitar or sarod.
3. Notable Work: He is best known for his collaboration on the landmark 1967 album Call of the Valley, which he recorded alongside legendary musicians Pandit Shivkumar Sharma (santoor) and Pandit Hariprasad Chaurasia (flute). This album played a massive role in popularizing the slide guitar as a classical Indian instrument globally.

Therefore, Brij Bhushan Kabra was a master and expert of the Guitar in the field of Indian classical music.